1. Field of the Invention
The present invention relates to a method for forming a resist pattern and a method for manufacturing a semiconductor device.
2. Description of Related Art
To satisfy a demand for improvement of integration degree, a main stream of a light source used in an exposure process in the photolithography shifts from a mercury lamp for irradiating an i-ray to a KrF excimer laser, improvement of sensitivity of a resist is advanced in order to respond decrease in exposure intensity on a wafer surface during the exposure, and a resist of a chemical amplification type has been often used.
The resist of the chemical amplification type includes a base resin and a PAG (Photo Acid Generator), and a resist of a negative type further includes a cross-linker. A hydrogen ion (acid) is generated from the PAG with exposure energy at the exposure process. In a Post Exposure Bake process after the exposure (hereinafter referred to as a PEB process), the hydrogen ion attacks a protective group in a base resin to cause a deprotection reaction in a case where the resist is a positive type (in a case of a positive resist), and the hydrogen ion acts on the cross-linker to cause a cross-linking reaction of the base resin in a case where the resist is the negative type (in a case of a negative resist). In both cases, the hydrogen ion is newly generated in the PEB process, the new hydrogen ion causes a next deprotection reaction or cross-linking reaction, and thereby sensitivity of the resist is improved.
As shown in FIG. 5, in a general photolithography process using the resist of the chemical amplification type, the resist is firstly applied on a wafer (a resist application process, step S101), and the resist is baked after the application, for example, at 80 to 135° C. (a prebaking process, step S102). Moreover, after that, the exposure process (step S103), the FEB process (step S104), and a development process (step S105) are carried out in this order to form the resist in a predetermined pattern shape.
Here, as shown in FIG. 2, in the prebaking process at step S102, a plurality of ceramic balls 2 are provided on a hot plate 1 of a prebaking machine (not entirely illustrated), and the wafer 3 is heated in a state where the wafer 3 is placed on the ceramic balls 2.
However, since the wafer 3 is warped in the heating, baking temperatures sometimes vary between a central portion and a circumferential portion on the wafer 3. The warping of the wafer 3 and difference of the baking temperatures caused by the warping become prominent in a case where the wafer 3 is extremely thin or in a case where a metal film, whose linear expansion coefficient is considerably different from that of Si, is formed only on one surface of the wafer 3.
When the baking temperature varies between the central portion and the circumferential portion, sensitivity of the resist (Eth) varies between the central portion and the circumferential portion. For this reason, when a whole surface of the wafer 3 is exposed by an even exposure amount, a pattern size of the resist varies between the central portion and the circumferential portion. Meanwhile, the sensitivity (Eth) is represented by minimum exposure energy (mJ/cm2) at which a remainder of the resist film is not produced (a rate of remaining film becomes 0%) in a case where the development is carried out under a fixed condition.
As described above, in the prebaking process, there is a case where the heating temperature becomes uneven on the surface of the wafer 3 by the warping, and thereby the pattern size of the resist varies on the surface of the wafer 3. In addition, the same phenomenon occurs also in the PEB process.
Document 1 (Japanese patent publication JP-2006-135080A) discloses a technique for not carrying out the exposure on the whole surface of the wafer at an even exposure amount but carrying out the exposure by adjusting the exposure amount for each of a plurality of regions on the wafer surface. Specifically, the exposure amount is adequately adjusted on the surface of the wafer by changing the exposure amount for each exposing shot. In this manner, the variation in the pattern size is suppressed, which is attributed to the temperature unevenness caused by the warping in the PEB process (so-called a variable exposure technique). Also in document 2 (Japanese patent publication JP-A-Heisei 10-172889), a technique similar to that of document 1 is disclosed.